BLYTHE, Calif. (AP) â€” A passenger bus flipped onto its side and slid down an embankment after a jackknifed big rig spilled steel pipes across Interstate 10 in Southern California, leaving 4 people dead and several more seriously injured, authorities said.

The crash occurred around 2:15 a.m. Wednesday just west of Blythe near the Arizona border when a tractor trailer traveling east jackknifed onto the median and spilled its load of pipes into all lanes in both directions, said Terri Kasinga of the California Department of Transportation.

The westbound bus either swerved to avoid the pipes or struck them and overturned, sliding across the shoulder and 50 feet down a slight embankment, Kasinga said. The bus, with 32 confirmed passengers, was traveling from El Paso to Los Angeles.

Four vehicles in the eastbound lanes also crashed and several people were transported to hospitals in unknown conditions.

Four people from the bus were killed, Kasinga said.

Emergency helicopters transported several passengers with critical injuries to hospitals.

All lanes of the interstate will be closed until the afternoon, and possibly longer, Kasinga said. Traffic was backed up for miles in both directions and drivers on the main artery through the Lower Colorado River Valley were urged to find alternate routes.

The bus, operated by El Paso-Los Angeles Limousine Express, Inc., had switched drivers in Phoenix, Kasinga said. It was not known if the driver was among the victims. The company sent smaller charter buses to transport the uninjured passengers.

A message seeking comment from the bus company was not immediately returned.

The California Highway Patrol was investigating and will try to determine what caused the big rig to jackknife.

Blythe is a city of about 25,000 people in eastern Riverside County, about 225 miles east of downtown Los Angeles.